A heat stable, whey-containing nutritional beverage is known from WO 2006/119064, which describes a novel aseptic beverage and pudding comprising at least 2% whey, and a process for manufacture thereof.
There is presently an increasing demand for high-protein preparations as beverages, especially those with low carbohydrate contents. More particularly, this demand encompasses beverages having high levels of whey protein. Whey protein is considered an excellent source of protein for medical nutrition because of its easy digestion (“fast protein”) and its favourable amino acid composition. Whey protein has an excellent amino acid score of 1.2, contains a high amount of essential amino acids (590 mg/g), comprising among others 480 mg/g branched chain amino acids (leucine, valine and isoleucine) and 26 mg/g tryptophan. The cysteine content is higher than in most other food proteins (32 mg/g). The efficacy of whey protein in improving protein synthesis in elderly subjects was shown in several clinical studies [M. Dangin et al., J. Physiol 549 (Pt 2): 635-644, 2003, D. Paddon-Jones et al., Exp. Gerontol. 41 (2):215-219, 2006.]. Indications exist that whey protein improves insulin response in type II diabetics after carbohydrate ingestion [A. H. Enid et al., Am. J. Clin. Nutr. 82 (1): 69-75, 2005].
Due to its high cysteine content, whey protein may positively influence the plasma level of glutathione in subjects exposed to severe oxidative stress, which was shown in patients with advanced HIV infection [P. Micke, K. M. et al., Eur. J. Nutr. 41 (1): 12-18, 2000]. Glutathione is a crucial endogenous antioxidant, radical scavenger and detoxicant, the metabolism of which is compromised at many clinical and sub-clinical conditions and at higher age. The favourable tryptophan/LNAA (large neutral amino acids) ratio of whey protein is supposed to positively affect the brain serotonin level in stress-prone subjects [C. R. Markus et al., American Journal of Clinical Nutrition 71 (6):1536-1544, 2000]. Whey protein is also appreciated in bodybuilding and it is very popular in both weight loss nutrition and sports nutrition. In addition to containing whey protein, high-protein beverages often contain many vitamins and minerals. In contrast to whey-containing products for healthy people, which are often formulated as refreshing drinks with a low pH, products used for medical nutrition usually require a final product pH in the neutral range and heat treatment. At neutral pH, whey proteins are not heat-stable.
Therefore existing shelf-stable liquid products that contain whey proteins usually contain only a very small quantity of this highly nutritional protein, because of the seemingly insurmountable problems caused by the whey proteins during a conventional sterilisation manufacturing process.
In order to obtain a long shelf life of beverages of 12 months or longer, sterilisation is the preferred heat treatment, in particular as a so-called UHT treatment. Typically in the UHT process, the product is indirectly heated to 135° C.-140° C. by means of heating coils, and held at this temperature for 6-10 seconds, or heated directly by live steam under pressure at 140° C.-150° C. for 2-4 seconds, followed by aseptic packaging. Such a process allows an extension of shelf life to 12 months or more, though at the cost of some change in organoleptic properties. A further possibility is the so-called retort process, wherein the product is completely sterilised by sealing in cans which are then heated in an autoclave at 110° C.-130° C. for 10-20 minutes. However, as will be evident to the skilled artisan, the retort process can cause heat damage to, or even destroy, sensitive ingredients and may result in unacceptable organoleptic and physical changes.
This is especially a major problem for beverages containing whey protein, more particular at whey protein concentrations of over 2% w/w at neutral pH. Because of the high temperatures involved in sterilisation processes (either UHT or batch), the whey-containing beverages tend to lose consistency, and protein may sediment out in the form of lumps or aggregates at the bottom of the container. A further problem is that the exposure of the proteins to heat may also cause gelation, resulting in increased viscosity such that beverages become almost pudding-like in consistency. Furthermore, the presence of more than about 2% whey protein in a beverage results in a short shelf life and unsatisfactory organoleptic properties characterised by unpalatable taste, gelation, sedimentation and grittiness or powdery texture in beverages.
In the case of liquid medical food preparations, the risk of microbial contamination needs to be eliminated. Such beverages need to be subjected to thermal sterilisation processes in order to eliminate possible pathogens. However, as explained above, this severe thermal treatment has a negative effect on the organoleptic and physical quality of the liquid preparation in case high amounts of thermally sensitive proteins such as whey proteins are present.
As explained before, there is a need for a heat stable, nutritional beverage having a high content of whey proteins with a shelf life of at least 12 months.
In the art, no satisfactory solutions have so far been brought forward. WO 06/119064 proposes to use a specific homogenisation process to solve the severe problem of aggregation of the whey proteins, but this is only partially successful as the highest whey protein concentration disclosed is 4.1% w/w.
Boye and Alli (Thermal denaturation of mixtures of α-lactalbumin and β-lacto-globulin: A differential scanning calorimetric study, Food Research International 33 (2000), 673-682) describe heat treatments of (mixtures of) 2 purified whey proteins at very high protein concentration (40 wt. %) in the presence of a high sugar concentration (20 wt. %), and there is no relation to a beverage.